This invention relates to sealing machines, and in particular to a platen seal for unsupported plastic films in a sealing machine, the platen being used to form a longitudinal seal in the plastic film.
Vertical form fill and seal machines are well known. For example, U.S. Pat. No. 4,288,965, the disclosure of which is incorporated herein by reference, is directed to such a machine. The invention disclosed herein is intended to be utilized in connection with a vertical form fill and seal machine, although it can also be used in other applications.
For many years manufacturers of vertical form fill and seal (VFFS) equipment have been concerned about seal quality. This is particularly important when materials such as unsupported polyethylene are used as the packaging material.
To seal unsupported polyethylene, or any material which reverts to a molten state as it is being sealed, the seal area is sealed with a heated sealing bar which is placed in contact with the film at a given pressure for a preset amount of time to have the sealing process take place by fusing the material together into a solid mass. Three key parameters determine the quality of the seal:
1) Temperature PA1 2) Time PA1 3) Pressure.
Once the seal has been completed by melting the film, it is necessary to cool the material to solidify the seal and recapture the lost strength before applying any pressure on the seal area. This is generally done by blowing cooling air onto the seal area which supporting the film in a relaxed condition, or contacting the seal area with a cooling bar which extracts the excess heat by conduction rather than convection.
Several forms of back seal sealing systems have been used over the years. One of these involves the use of a hot air blast directed on the moving film to seal the overlapped film. This works well when the film is in constant movement, but for intermittent motion, as used with the VFFS equipment, seal continuity is a serious problem with these systems. A system of this type is described in U.S. Pat. No. 5,466,326 for a continuous motion machine, the disclosure of which is incorporated therein by reference.
For many years, the traditional method of making back seals on a VFFS machine has been to use a heated bar referred to in the industry as a platen. This platen is heated to a specific temperature which is controlled by a closed loop temperature controller.
The time aspect of the seal is controlled by a timer or timer integral to the main controller.
The platen is generally positioned using a pneumatic cylinder in conjunction with some form of mechanical linkage. This provides a relatively constant pressure. As a result, time, temperature and pressure are controlled to provide the desired seal.
The platen is always made longer than the film pull to avoid any gaps in the back seals. In the event the bag length is longer than the length of the platen, a double pull is made to insure an adequate overlap in platen sealing. This works extremely well for materials which tolerate a double sealing operation.
However, materials such as polyethylene have a tendency to exhibit burn through if two sealing operations are attempted in quick succession. This causes a loss in seal quality which is unacceptable in many applications.
To attempt to resolve the burn through which still maintaining a back seal overlap, the platen has been skewed slightly in many applications. This allows an overlap without double sealing. The only problem is that there is a gap between seals which in most cases is closed by the end seal. This solution is an answer for about 95% of the applications, but back seal leaks can still occur if the gap is not covered by the end seal.
If this is not an alternative, another method must be found to insure a continuous seal with a minimal amount of overlap. The device according to the present invention fulfills this requirement in a unique and novel way.